THE CONCEPT OF AN EQUILIBRIUM SURFACE-APPLIED TO PARTICLE SOURCES ANDCONTAMINANT DISTRIBUTIONS IN ESTUARINE SEDIMENTS

Studies have shown that many chemically-reactive contaminants become a ssociated with fine particles in coastal waters and that the rate, pat tern, and extent of contaminant accumulation within estuarine systems are extremely variable. In this paper, we briefly review our findings concerning the accumulation patterns of contaminants in several estuar ine systems along the eastern coastline of the United States, and have applied a well-established concept in geology, that is ''an equilibri um profile,'' to explain the observed large variations in these patter ns. We show that fine-particle deposition is the most important factor affecting contaminant accumulation in estuarine areas, and that accum ulation patterns are governed by physical processes acting to establis h or maintain a sediment surface in dynamic equilibrium with respect t o sea level, river discharge, tidal currents, and wave activity. Net l ong-term particle and particle-associated contaminant accumulation are negligible in areas where the sediment surface has attained ''dynamic equilibrium'' with the hydraulic regime. Contaminant accumulation in these areas primarily occurs by the exchange of contaminant-poor sedim entary particles with contaminant-rich suspended particles during phys ical or biological mixing of the surface sediment. Virtually the entir e estuarine particulate and contaminant load bypasses these ''equilibr ium'' areas to accumulate at extremely rapid rates in relatively small areas that are temporally out of equilibrium as a result of natural p rocesses or human activities. These relatively small areas serve as ma jor sinks for particles from riverine and marine sources, and for biog enic carbon formed in situ within estuaries or on the inner shelf.